The present invention relates to a method and an arrangement for determining angular velocities with the aid of an optical fiber ring interferometer, wherein light waves passing through an optical fiber of the ring interferometer are phase modulated and the output signal of the ring interferometer is evaluated after an analog/digital conversion in order to determine the angular velocity.
Such a method and an arrangement for implementing the method are disclosed in U.S. Pat. No. 4,549,806 which issued on Oct. 29, 1985, the subject matter of which being incorporated herein by reference. The optical fiber of this ring interferometer serves to propagate two light waves which move in opposite directions and which interfere with one another when they exit from the optical fiber. The interference depends on the angular velocity with which the optical fiber in the form of at least one turn is rotated. The angular velocity is proportional to the phase difference between the two light waves that have passed through the optical fiber in opposite directions. This phase difference, the so-called Sagnac phase shift, can be determined from the amplitudes of the spectral lines of the interference light exiting from the optical fiber.
In order to obtain an optical fiber ring interferometer output signal which is suitable for a determination of the Sagnac phase, light waves exiting from one end of the optical fiber disclosed in U.S. Pat. No. 4,549,806 are phase modulated and combined to produce an analog output signal. The evaluation of the analog output signal, in order to determine the Sagnac phase, is advantageously effected by means of a digital signal processing circuit. Since the analog output signal has an unfavorably high frequency position for the subsequent analog/digital conversion, due to the required high phase modulation frequency, U.S. Pat. No. 4,546,806 provides means for either pulsing the power of light fed into the optical fiber ring interferometer at a suitable frequency or reducing the output signal by means of a mixer to a lower frequency position before the analog/digital conversion. However, these solutions entail additional circuitry expenditures. Another drawback of employing a mixer to reduce the output signal frequency is that undesirable mixing products result which must be suppressed by additional analog filtering in order to meet the sampling theorem and thus, negate any undue spectral convolution products which would greatly falsify the signal to be evaluated.